Carbon dots (CDs) are a class of carbon-based nanoparticles that comprise discrete carbogenic nanoparticles with sizes below 10 nm. CDs have emerged as versatile fluorescent nanoparticles possessing unique features such as high quantum yields, nontoxicity, nonblinking, high photostability and vast accessibility, with strong potential to be applied in bioimaging, sensing and optoelectronic devices. CDs can be synthesized through a number of methods including laser ablation, electrochemical exfoliation, carrier-supported aqueous route, combustion route, hot injection, hydrothermal treatment, microwave treatment, and so forth. These methods generally result in hydrophilic CDs with abundant —COOH and —OH groups on the surface of the CD, which are amenable for further functionalization.
Various functionalised CDs have been demonstrated as effective fluorescence probes for the detection of copper ions, ferric ions, silver ions, as well as mercury ions in water. The presence of the cation analyte quenches the CD fluorescence with the fluorescence intensity being proportional to the concentration of analytes, most likely due to the effect of electron transfer. In terms of Hg2+ sensing, functionalised CDs provide many advantages such as high sensitivity, better water solubility, economic and green synthesis routes, convenient detection procedure, and compatibility with various sensing platforms such as optical fibre devices.
Notwithstanding the promising features of many functionalised CDs, there is still a need to improve the various properties of functionalised CDs in order for them to be employed in commercial and industrial applications. These improvements include simpler and more efficient synthetic methods, improved optical properties such as tunable emission bands and enhanced quantum yield (QY), heightened sensitivity, specificity and durability in complex fluids, such as wastewater effluent. Wastewater effluent, for example, contains large amounts of organic matter, bacteria and viruses; with an inherently high fluorescence background. Moreover, organic and biological matter is prone to interact or contaminate nanoparticle surfaces, representing a challenging sample type for fluorescence probes.
Despite being one of the most toxic heavy metal ions, mercury (Hg2+) ion is widespread and widely used in industry, causing serious environmental and health concerns. With the maximum contamination limit for Hg2+ in drinking water set at 2 ppb (˜10 nM) by the United States Environmental Protection Agency, detection and remediation of Hg2+ in water has always been a high priority area in environment and public health. The increased necessity and practice in wastewater recycling in recent years has further increased the urgency of developing facile and accurate Hg2+ detection methods. The safety and public acceptance of the use of recycled wastewater demands monitoring of the presence and concentration of toxic chemicals, such as Hg2+, in both wastewater and recycled water.
Many analytical methods for Hg2+ detection have been developed including surface-enhanced Raman scattering (SERS) technique, surface plasmon resonances, inductively coupled plasma mass spectrometry, fluorescence chemosensors, electrochemical methods, and so forth. Among them, fluorescence-based sensing probes are desirable owing to several of their advantageous characteristics including high sensitivity, fast response, non-destructiveness and convenient operations. Most of the known Hg2+ fluorescence probes are metal-based, such as gold and silver nanoparticles and nanowires. Organic molecules and semiconductor quantum dots have also been applied as fluorescence probes for Hg2+ detection. The above fluorescence probes, however, possess several disadvantages which greatly limit their practical application, such as high production costs, toxicity of the probe materials, poor stability and complex synthesis procedures. Therefore, new Hg2+ fluorescent probes that can overcome the above limitations are highly desirable. Any references to background art do not constitute an admission that the art forms a part of the common general knowledge of a person of ordinary skill in the art. The above references are also not intended to limit the application of the process and the system as disclosed herein.